The present invention relates generally to a cooling mechanism, and more particularly to a cooling mechanism used for an optical element in an exposure apparatus that manufactures devices, such as a single crystal substrate for a semiconductor wafer and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for a cooling mechanism for cooling an optical element in an exposure apparatus that uses as exposure light ultraviolet (“UV”) light and extreme ultraviolet (“EUV”) light.
There has conventionally been employed a reduction projection exposure apparatus that uses a projection optical system to project a circuit pattern on a mask or a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing such fine semiconductor devices as a semiconductor memory and a logic circuit in the photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Recent demands for finer semiconductor devices have promoted a shorter wavelength of ultraviolet light from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
The lithography using the UV light has the limit to satisfy the imminent demands for finer semiconductor devices. Accordingly, a reduction projection exposure apparatus has been developed which uses EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet light (referred to as an “EUV exposure apparatus” hereinafter) to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
As the exposure light has a shorter wavelength, its absorption remarkably increases in a material, and becomes incompatible with use a refraction element or lens for visible light and ultraviolet light. No glass material is compatible with a EUV light's wavelength, and a reflection-type or cataoptric optical system is used which utilizes only a reflective element or multilayer mirror. A reticule also uses a reflection mask that uses an absorber on a mirror to form a pattern to be transferred.
The mirror cannot reflect all of the exposure light, but absorbs 30% or greater of the exposure light. The energy of most of the absorbed exposure light turns to residual heat, and would possibly deform a mirror's surface, causing deteriorated optical performance (in particular, imaging performance). Accordingly, a mirror is made of ceramics, such as SiC, SiN, AlN and Al2O3, which has such a small coefficient of linear thermal expansion as about 1 ppm to about 10 ppm to reduce changes of a mirror shape.
The EUV exposure apparatus is used for exposure of a circuit pattern of 0.1 μm or smaller, and is required to have such strict critical dimension precision that a projection optical system's mirror permits a deformable surface shape of only about several nanometers. A shape error budget σ (rms value) permitted to a mirror is given in the Marechal's criterion as Equation 1 below, where λ is a wavelength of EUV light, and n is the number of mirrors in the projection optical system:
                    σ        =                  λ                      28            ×                          n                                                          (        1        )            
For example, where the EVU light has a wavelength of 13 nm and the projection optical system uses four mirrors, the shape error budget σ becomes 0.23 nm. As the exposure heats a mirror that has such a small coefficient of linear thermal expansion as several ppm in the projection optical system, a deformed surface possibly exceeds the permissible shape error, and makes imaging performance insufficient, or the resolution and contract become too low to transfer fine patterns.
Accordingly, as shown in FIG. 13, there has been proposed a method for connecting a conduit 1200 to a channel 1110 formed in a mirror holder 1100 that holds a mirror 1000, and for cooling the mirror 1000 via the mirror holder 1100 by introducing coolant, such as water, into the channel 1110 (see, for example, Japanese Patent Application Publication No. 10-209036). FIG. 13 is a schematic sectional view of the mirror 1000 for explaining a conventional mirror cooling method.
A wafer and a reticle similarly absorb exposure light and deform due to residual heats. Accordingly, as shown in FIG. 12, the wafer and reticle are cooled by connecting a conduit 1600 to a channel 1510 in the chuck 1500 that holds the reticle and the wafer, and by introducing coolant, such as water, into the channel 1510. FIG. 12 is a schematic sectional view of the chuck 1500 for explaining a conventional method for cooling the reticle and the wafer.
The cooling method shown in FIG. 13 can cool the mirror holder 1100, but cannot cools the mirror 1000 sufficiently, because the mirror holder 1100 cannot be adhered onto the mirror 1000 for control over the mirror 1000's orientations. It is conceivable to form a channel in the mirror 1000 and cool it directly, the method for cooling the reticle and the wafer shown in FIG. 12 supports a fine movement stage 1520 mounted with the chuck 1500, using only a spring 1530 with weak rigidity. Therefore, vibrations resulting from vortexes, pulsations, etc. in the channel 1510 and conduit 1600 deteriorates chuck 1500's positional stability. In addition, the heavy channel 1510 and conduit 1600 that flow water restrain movements of the fine movement stage 1520. In other words, although it is expected that, as shown in FIG. 14, a laser interferometer 1700 detects a mirror 1000 attached to the fine movement stage 1520 to precisely control a position of the fine movement stage 1520 (or positions of the reticle and the wafer), the fine movement stage 1520 has such a bad positional control response that the fine movement stage 1520 cannot move its position quickly or restrain high-frequency vibrations. Here, FIG. 14 is a schematic sectional view of the chuck 1500 for explaining positioning of the reticle and the wafer.
It is conceivable to use heat conductive gas, such as helium, for coolant instead of water to prevent generations of vortexes, pulsations, etc. The efficient cooling of the mirror and wafer requires the flow of low-temperature gas. Nevertheless, the temperature of the gas with small heat capacity easily rises before the gas reaches the mirror holder and the fine movement stage, and the gas cannot cool them sufficiently.
Japanese Patent Application Publication No. 10-209036 also proposes a method for cooling a wafer using adiabatic expansion of the gas in an aperture between the wafer and the chuck. However, the gas between the wafer and the chuck reduces the chuck's absorptive force to the wafer. For example, when the chuck has an absorptive force of about P1 kgf/cm2 without gas, the chuck's absorptive force is reduced to (P1−P2) kgf/cm2 if the gas exists between the wafer and the chuck with gas pressure of P2 kgf/cm2. An electrostatic chuck that has a pin-shaped surface to reduce a probability of adhesions of particles usually has an absorptive force of about 100 gf/cm2, and cannot use the gas having pressure above this value.
The heat quantity removable by the gas is determined by specific heat times density times flow of the gas. The low gas pressure means the low density, and the small removable heat quantity requires a long cooling time or results in insufficient cooling ability. When the pressure decreases as the exposure starts, the wafer's temperature can decrease due to adiabatic expansion. However, this configuration cannot increase the pressure so that the gas pressure becomes zero or nearly zero and the cooling ability cannot be maintained.